1. Field of the Invention
This invention relates to ink jet printing, and more particularly, to coatings for nozzle-containing faces of printheads used in ink jet printing and methods of applying the coatings.
2. Description of Related Art
In ink jet printing, a printhead is usually provided having one or more ink-filled channels communicating with an ink supply chamber at one end and having an opening at the opposite end, referred to as a nozzle. These printheads form images on a recording medium such as paper by expelling droplets of ink from the nozzles onto the recording medium. The ink forms a meniscus at each nozzle prior to being expelled in the form of a droplet. After a droplet is expelled, additional ink surges to the nozzle to reform the meniscus. An important property of a high quality printhead array is good jet directionality. This ensures that ink droplets can be placed precisely where desired on the print document. Poor jet directional accuracy leads to the generation of deformed characters and visually objectionable banding in half tone pictorial images.
A major source of ink jet misdirection is associated with improper wetting of the front face of the printhead which contains the array of nozzles. One factor which adversely affects jet directional accuracy is the interaction of ink accumulating on the front face of the printhead array with the ejected droplets. Ink may accumulate on the printhead face either from overflow during the refill surge of ink or from the spatter of small satellite droplets during the process of expelling droplets from the printhead. When the accumulating ink on the front face makes contact with ink in the channel (and in particular with the ink meniscus at the nozzle orifice) it distorts the ink meniscus resulting in an imbalance of the forces acting on the egressing droplet which in turn leads to jet misdirection. This wetting phenomenon becomes more troublesome after extensive use as the array face oxidizes or becomes covered with a dried ink film. This leads to a gradual deterioration of the image quality that the printhead is capable of generating. In order to retain good ink jet directionality, wetting of the front face desirably is suppressed. Alternatively, if wetting could be controlled in a predictable, uniform manner, jet misdirection would not be a problem. However, uniform wetting is difficult to achieve and maintain.
In thermal ink jet printing, a thermal energy generator, usually a resistor, is located in the channels near the nozzles a predetermined distance therefrom. The resistors are individually addressed with a current pulse to momentarily vaporize the ink and form a bubble which expels an ink droplet. As the bubble grows, the ink bulges from the nozzle and is contained by the surface tension of the ink as a meniscus. The rapidly expanding vapor bubble pushes the column of ink filling the channel towards the nozzle. At the end of the current pulse the heater rapidly cools and the vapor bubble begins to collapse. However, because of inertia, most of the column of ink that received an impulse from the exploding bubble continues its forward motion and is ejected from the nozzle as an ink drop. As the bubble begins to collapse, the ink still in the channel between the nozzle and bubble starts to move towards the collapsing bubble, causing a volumetric contraction of the ink at the nozzle and resulting in the separation of the bulging ink as a droplet. The acceleration of the ink out of the nozzle while the bubble is growing provides the momentum and velocity of the droplet in a substantially straight line direction towards a recording medium, such as paper. The collection of ink on the nozzle-containing face of thermal ink-jet printheads causes all of the problems discussed above.
Ink jet printheads include an array of nozzles and may be formed out of silicon wafers using orientation dependent etching (ODE) techniques. The use of silicon wafers is advantageous because ODE techniques can form structures, such as nozzles, on silicon wafers in a highly precise manner. Moreover, these structures can be fabricated efficiently at low cost. The resulting nozzles are generally triangular in cross-section. Thermal ink jet printheads made by using the above-mentioned ODE techniques are typically comprised of a channel plate which contains a plurality of nozzle-defining channels located on a lower surface thereof bonded to a heater plate having a plurality of resistive heater elements formed on an upper surface thereof and arranged so that a heater element is located in each channel. The upper surface of the heater plate typically includes an insulative layer which is patterned to form recesses exposing the individual heating elements. This insulative layer is referred to as a "pit layer" and is sandwiched between the channel plate and heater plate so that the nozzle-containing front face has three layers: the channel plate, the pit layer and the heater plate. For examples of printheads employing this construction, see U.S. Pat. Nos. 4,774,530 to Hawkins and 4,829,324 to Drake et al, the disclosures of which are herein incorporated by reference.
These heater and channel plates are typically formed from silicon. The pit layer sandwiched between the heater and channel plates, however, is formed from a polymer, typically polyimide. Since the front face of the printhead is made from different materials, a coating material , such as a water-repellent material, will not adhere equally well to these different materials, resulting in a coating which is not uniformly ink-repellent. Thus, it is difficult to provide a surface coating which is uniformly ink-repellent in ink jet printheads formed from multiple layers.
Additionally, these printers typically use an ink which contains a glycol and water. Glycols and other similar materials are referred to as humectants, which are substances which promote the retention of moisture. For a coating material to be effective for any length of time, it must both repel and be resistant to glycol-containing inks.
Further, it is difficult to apply a coating to the face of an ink jet nozzle. While it is desirable to suppress the wetting property of the nozzle jet surface, it is undesirable to allow any coating material to enter the channels of the nozzle. A key requirement for good directionality is that the interior channel walls not be coated. If the walls of the channels become coated with ink-repellent material, proper refill of the channel is inhibited. Refill of each channel depends on surface tension and must be completed in time for subsequent volleys of drops to be fired. If the refill process is not complete by the time the next drop is fired, the meniscus may not be flush with the outer edge of the nozzle orifice, resulting in misdirection. Further, an incompletely filled channel causes drop size variability which also leads to print quality degradation.
Uehara et al U.S. Pat. No. 4,368,476 discloses ink jet recording heads which are treated with a compound represented as RSiX.sub.3, wherein R is a fluorine containing group and X is halogen, hydroxyl or a hydrolyzable group. The ink jet recording head may contain a number of differing materials, and accordingly, it is difficult to provide uniform coating.
Diaz et al U.S. Pat. No. 4,643,948 discloses coatings for ink jet nozzles. An ink jet nozzle plate is coated with a film which comprises two ingredients. One ingredient is a partially fluorinated alkyl silane and the other ingredient is a perfluorinated alkane. The silane compound and the alkane compound are preferably deposited on the nozzle surface by direct exposure of the surfaces to radio frequency glow discharge. The Diaz et al reference does not disclose application of an ink-repellent material to a printhead made from silicon or that is compatible with glycol-containing based inks. Additionally, Diaz et al does not address any of the problems involved with applying a liquid-repellent material to a nozzle-containing face made from multiple materials.
Le et al U.S. Pat. No. 4,734,706 discloses a printhead for an ink jet printer having a protective membrane formed over the ink orifice. A viscoelastic and ink-immiscible fluid is used to form the membrane over the ink orifice. The membrane may comprise a silicone oil such as polydimethylsilicone polymers. The membrane lies in a plane perpendicular to the direction of emission of ink drops, and provides a barrier between the ink orifice and the external atmosphere, thus inhibiting evaporation of ink and the entry of contaminants. Wetting of the exterior surface of the ink jet head by the flow of ink through the ink orifice is also inhibited.
Miura et al U.S. Pat. No. 4,728,392 discloses an ink jet printer of the electro-pneumatic type wherein an inner surface of a front nozzle plate and an end face of a rear nozzle member may be coated with a thin layer of an ink-repellent material. The ink-repellent material may be an ethylene tetrafluoride resin such as Teflon, a trademark of du Pont, or a fluoride-containing polymer. Miura et al also discloses blowing air through a nozzle while an ink-repellent material is applied thereto to prevent clogging of the nozzle. The nozzle-containing face of Miura et al is made from one material.
Fujimura et al U.S. Pat. No. 4,751,532 discloses a thermal electrostatic ink jet recording head wherein thermal energy and an electrostatic field are applied to ink held between two plate members to cause the ink to be jetted out from an orifice defined by the plate members. Critical surface tensions must be satisfied to maintain a desired shape of the meniscus to provide good printing quality. Surfaces of the plate members are treated to provide different surface tensions. The surfaces may be treated with a silicone-type or fluorocarbon-type resin. Fujimura et al requires that an area surrounding the nozzle remains adherent to liquid and also does not address the problems which arise when a nozzle face is made from different materials.
Chandrashekhar et al U.S. Pat. No. 4,623,906 discloses a surface coating for ink jet nozzles. The coating includes a first layer of silicon nitride, an intermediate layer graded in composition, and a top-most layer of aluminum nitride. Chandrashekhar et al provide this structure to aid in adhering the low wettable, aluminum nitride layer to the nozzle-containing face which is made from glass or silicon. Chandrashekhar et al do not address the problem of coating a nozzle-face made from multiple, different materials or disclose any of the materials usable in the present invention for coating silicon.